Threaded Insert & Screw Boss Calculator
Boss and hole sizing for heat-set inserts — why the 2x rule is right, and the one refinement printed plastic adds to it.
Why 2× and not something else
The 2× boss rule is right, and it isn't inherited lazily
A boss is a thick-walled cylinder under internal pressure, and the hoop stress at the bore is (b² + a²)/(b² − a²) — which depends only on the ratio of the diameters, not on the material or the process. That's why the figure transfers from injection moulding unchanged, and it's why 2× sits exactly at the knee: dropping to 1.5× raises the stress 56%, while going up to 3× lowers it only 25% for nearly twice the material. Every step above two buys less than it costs; every step below costs more than it saves.
But printed plastic adds one thing moulding can't. A printed boss wall is made of whole extrusions, and at exactly 2× none of the common sizes lands on a whole number — an M3 wants 5.48 perimeters, an M4 6.67, an M5 7.62. The slicer then either overlaps the lines or leaves a void in the wall that's carrying the load. Round the boss so the wall is a whole number of extrusion widths: it moves the diameter a few per cent, deep inside the flat part of the stress curve, and removes the gap for nothing.
So: keep the moulding rule, then round it to whole perimeters. A wider line rounds more coarsely — up to about 9% on a small insert with a 0.6 mm nozzle, against 3 or 4 with a 0.4 — but the rounding can never move the wall by more than half a line width either way.
- Melt the insert in, don't press it. Cold-pressing splits printed bosses along a layer line more often than not — printed parts are weakest exactly where the hoop stress is highest. A soldering iron with a proper insert tip, 20–40 °C above the material's printing temperature, going in slowly and straight.
- The hole matters more than the boss and has a narrower window. Too large and the insert spins with nothing you can do about it afterwards; too small and it splits the boss going in. Those figures are manufacturer-specific — the same nominal size from two suppliers can want holes 0.4 mm apart, and the datasheet wins over any table.
- Length-to-diameter under about 1.0 has little to grip. Where the load is high, two shorter inserts usually beat one long one, because the failure is in the plastic rather than the metal.
- Don't leave a boss standing alone off a thin wall. It concentrates all the load at its base, which is a layer boundary and therefore the weakest plane in the part. A gusset, a rib, or a corner where two walls carry it costs almost no material.
How to use
- Take the insert outside diameter and hole from the manufacturer datasheet.
- Size the boss at twice the insert diameter, then round to whole perimeters.
- Melt the insert in with a proper tip rather than pressing it.
- Tie the boss into a wall rather than leaving it standing alone.
Frequently asked questions
How big should a boss be for a heat-set insert?
Twice the insert outside diameter, then rounded so the wall is a whole number of extrusion widths. For a 4.6 mm M3 insert that is 9.2 mm from the raw rule and 8.8 mm rounded to five perimeters at 0.42 mm lines. The rounding moves the diameter a few per cent and the hoop stress by about the same, which is nothing where the curve is flat.
Why is the boss rule twice the insert diameter?
Because a boss is a thick-walled cylinder under internal pressure and the hoop stress at the bore is (b squared plus a squared) over (b squared minus a squared) — which depends only on the ratio of the diameters. Two times sits exactly at the knee of that curve: dropping to 1.5 raises the stress 56 per cent, while going up to 3 lowers it only 25 for nearly twice the material.
Does the moulding boss rule apply to 3D printing?
Unchanged, because it is pure geometry rather than a property of the process. The hoop stress ratio knows nothing about how the plastic got there. What printing adds is that the wall is built from whole extrusions, so the diameter should be rounded to a whole number of them — which moulding never has to think about.
Why round the boss diameter to whole perimeters?
Because at exactly twice the insert diameter, none of the common sizes lands on a whole number — an M3 wants 5.48 perimeters, an M4 6.67, an M5 7.62. The slicer then either overlaps the lines or leaves a void in the wall that is carrying the load. Rounding costs a few per cent of hoop stress and removes the gap entirely.
What hole size does a heat-set insert need?
Manufacturer-specific, and the datasheet beats any table. Typical figures are 3.0 mm for an M2, 4.0 for an M3 and 5.0 for an M4, but the same nominal size from two suppliers can want holes 0.4 mm apart. The window is narrower than the boss diameter and matters more — too large and the insert spins under torque, too small and it splits the boss going in.
Can I press a heat-set insert in cold?
It splits printed bosses along a layer line more often than not. The insert is designed for the plastic to flow into its knurls, which only happens with heat, and a printed part is weakest exactly where the hoop stress is highest. Use a soldering iron with a proper insert tip at roughly twenty to forty degrees above the printing temperature, going in slowly and straight.
Why did my printed boss crack when I installed the insert?
Usually one of three things: the hole was undersized, the insert went in cold, or the boss wall was thinner than about twice the insert diameter. All three raise the hoop stress at the bore, and the bore is surrounded by layer boundaries that are the weakest planes in the part. Check the hole against the datasheet first, since that is the cheapest thing to be wrong about.
How deep should the hole for an insert be?
Around twenty per cent deeper than the insert, so it cannot bottom out before it seats flush. An insert that hits the bottom stops going in while the shoulder is still proud, and forcing it from there is how bosses split. The extra depth costs nothing and removes the failure mode entirely.
How much pull-out strength does an insert have?
It depends on the engagement area, which is the insert circumference times its length — about 82 square millimetres for a standard M3 and 143 for an M4. A length-to-diameter ratio under about 1.0 has very little to grip. Where the load is high, two shorter inserts usually beat one long one, because the failure is in the plastic rather than in the metal.
Should a screw boss stand alone on a wall?
Not if it takes real load. A boss standing proud of a thin wall concentrates everything at its base, which is a layer boundary and therefore the weakest plane in the part — it tears out of the panel with the insert still in it. A gusset or two, a rib back to a wall, or a position in a corner where two walls carry it costs almost no material and changes the failure mode completely.
Does line width change the boss size?
It changes how coarsely the boss rounds. The wall has to be a whole number of extrusions, so a 0.6 mm nozzle steps in bigger increments than a 0.4 — up to about nine per cent of diameter on a small insert against three or four. The rounding can never move the wall by more than half a line width either way, so a wider line is not worse, just less precise.
Are heat-set inserts better than printed threads?
For anything that will be assembled more than once, considerably. A printed thread in PLA strips after a handful of cycles and cannot be repaired; an insert gives a metal thread that outlasts the part. Printed threads earn their place where the fastener is a one-off, where metal is unwelcome, or where the thread is large enough that its own cross-section is substantial.
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